#include <iostream>
using namespace std;
int main(){
	int n, m;
	cin >> n >> m;
	if (n % 2 == 1){
		if (m >= n){
			cout << 1 << endl;
		} else {
			cout << -1 << endl;
		}
	} else {
		if (m >= n){
			cout << 1 << endl;
		} else if (m * 2 >= n){
			cout << 2 << endl;
		} else {
			cout << -1 << endl;
		}
	}
}